1. Field of the Invention
The present invention relates to a master station to be used in a communications system and an access control method. More particularly, the present invention relates to an access control method which is to be used by a plurality of communications system sharing the same channel and which prevents interferences between the communications systems.
2. Description of the Background Art
As a technique for reducing interferences occurring between a plurality of communications system sharing the same channel, access control methods for reducing the influences of interference waves by using transmission power control have been proposed, as disclosed in, e.g., Japanese Laid-Open Patent Publication No. 2002-198834 and Japanese Laid-Open Patent Publication No. 2003-37556.
Japanese Laid-Open Patent Publication No. 2002-198834 discloses a method where an attenuator is provided in a base station for attenuating signal power and interference power, while utilizing the transmission power of a transmitter in a terminal station to ensure that the power level of a wireless signal to be input to a receiver attains a reference level.
On the other hand, Japanese Laid-Open Patent Publication No. 2003-37556 discloses a method in which a base station which has detected interference waves issues interference information to another base station which is transmitting the interference waves via a LAN network, and the other base station having received the notification lowers its transmission power based on the interference information.
However, in the case where the above-described communications system is a power line communications system, due to the characteristics of the power line transmission path, signal attenuation within a device's own communications system may far exceed the signal attenuation which causes interference in another communications system, depending on the configuration of the devices which are connected to the network. In other words, in the case where the technique of Japanese Laid-Open Patent Publication No. 2002-198834 or Japanese Laid-Open Patent Publication No. 2003-37556 is applied to a power line communications system, if interference between communications systems is prevented through power control, some of the devices in the system may become unable to perform communications with other devices due to a reduced signal intensity, depending on the device configuration. In the case of a wireless communication, too, a similar phenomenon may occur in that an attenuation in the signal intensity due to shielding obstacles may cause a drastic drop in the signal intensity despite a small physical distance.
With such conventional structures, it is impossible, through transmission power control, to suppress interferences for other communications systems while maintaining a good communication quality within a device's own communications system. Therefore, there has been a problem in that the throughput in each communications system is greatly deteriorated due to interferences between the communications systems, and that it is difficult to control the communication band.